The present invention relates to a picture compressing and restoring system and a method for forming a pattern to be recorded inside a spatial light modulator. More particularly, it relates to a picture compressing and restoring system capable of optically compressing and restoring a picture, and which is applicable to image processing e.g., picture phones, TV conferencing etc.
Today's information-oriented society requires the transmission and storage of pictorial data, including a large amount of information, as the most important function of the information age. In processing digitized still-pictures and data it is very difficult to record and transmit a large amount of pictorial data via the existing storage and digital networks. For example, a picture of an A4-sized format includes information of about 26 Mbytes (300 dpi) and one picture on a TV display contains the data of about 30 Mbytes. Accordingly, pictorial data must be compressed by an encoding device prior to recording and transmitting it.
A variety of systems have been proposed for encoding pictorial data and for standardizing the encoding system. Efforts have been made to make the existing media mutually connectable and compatible. In December of 1990, CCITT recommended the system H.261 for encoding the moving pictures of picture phones and at TV conferences. At the end of 1992 JPEG (Joint Photographic Experts Group), a joint session of CCITT and ISO (International Organization of Standardization) recommended that the colored still picture-encoding system be standardized. MPEG (Moving picture Experts Group) of ISO is now in examining systems for encoding moving pictures to be played on VTR systems, and has standardized the encoding system MPEG by which data transmission is carried out at the rate of not more than 1.5 Mbits per second.
DCT (discrete cosine transform) is explained as follows: (The detail of DCT is described in, for example, a handbook on television and pictorial information engineering published by the Ohm Company, Japan, Nov. 30, 1990, pp.115-133, pp.409-413.)
An analog picture is digitized through standardizing and quantizing processings. The standardizing operation is to array picture elements (sampling points) on the picture to get a representation of a variety of luminance levels of the arrayed picture elements, and a quantizing operation to transform the luminous value of each picture element into the corresponding one of eight luminance levels previously created. The compression of the digital pictorial data may be accomplished by any of two methods: the first one is a reversible encoding method allowing for the complete reproduction of the initial digital picture from the compressed data, and the other one is an irreversible encoding method permitting a certain degree of image distortion at the reproduction end of the initial digital picture from the compressed data. The irreversible coding method can compress the data more densely than can the reversible method. The typical make up of the irreversible coding system is such that a digital picture is put into a preprocessing unit wherein it is subjected to filtering for reducing its redundancy and then it is coarsely quantized according to the required picture's quality and coefficiency of the data's compression. The quantized output is reversibly coded by, for example, the Huffman encoding method.
The most generally used technique for creating irreversible coding is an anticipatory coding or conversion coding. The conversion coding is effective in the case where a reproduced picture may have a relatively low quality i.e. sampled values are converted to orthogonal coordinates and thereby effective coding is realized by eliminating the correlation of the sampled values. In practice, a picture is divided into small blocks comprising N picture elements x M lines each which are separately converted to corresponding values of an orthogonal coordinating system. The conversion coefficient varies with the frequency components in the range from a direct current to a high frequency. Power concentration is normally observed at the low-frequency components. Accordingly, many bits are distributed to the low-frequency components with due consideration of their visual characteristics while the high-frequency components are coarsely quantized with a lesser number of bits. This reduces the total number of bits for coding.
The orthogonal transformation is featured basically by the concentration of energy at low-frequency components and reflection of edge and line information on high-frequency components. Walsh-Hadamard transform and DCT relate to the orthogonal transformation. DCT is applied with two-dimensional compression of the pictorial data. Sequential transformation and inverse transformation of two-dimensional DCT are expressed respectively by the following equations (1),(2): ##EQU1##
An image reproducing system with an array of flat micro-lenses and Walsh's orthogonal expansion system of parallel images, which has Walsh's two-dimensional functional mask, are proposed by Akiba et al. in the "Fundamental study of a micro-optical image preprocessing system using an array of flat micro-lenses", Optics vol. 20, No.8, pp.507-513, August, 1991. The system proposed therein is intended to treat binary imaged character information and comprises of an array of flat micro-lenses, Walsh's functional mask and a CCD camera. An input image is used and a numerical character pattern is adhered to a diffusion plate to be illuminated by transmitting incoherent light from behind. One of the arrayed flat micro-lenses causes rays of light from the input image to meet together at the principal focus to form a real image. The pattern of the rays of light of this image then pass through a series of Walsh's functional masks and converge at a CCD camera and are recorded in an image memory of a computer wherein the light pattern is quantized by a summation on every pixel value. The Walsh's expansion coefficients are determined by subtracting the transmitted light intensity values corresponding to positive and negative portions of Walsh's functions.
The publication of unexamined patent application, JP, A, 57-10123 discloses a light source system which is basically composed of a light source, two acousto-optical modulators, a Fourier transforming lens and a detecting device and which is intended to make a cosine transformation by using a Fourier transform lens for light wave transformation.
In contrast to the above, a device according to the present invention, does not require the use of two acousto-optical modulators and a Fourier transform lens and is capable of executing a discrete cosine transformation according to ON- and summations by means of a converging lens and a mask (spatial filter) which substitutes a basic pattern of a discrete cosine transformation through a variety of transmission.
The publication of unexamined patent application, JP, A, 2-120917 discloses an optical hybrid arithmetic unit which is basically composed of a one-dimensional spatial optical modulator, a two-dimensional spatial light modulator (a spatial filter for discrete cosine transformation meaning a mask with a constant pattern not requiring rewriting), a one-dimensional photoelectric converter, a light source, an optical system for the two-dimensional spatial optical modulator and an optical system for converging to the one-dimensional photoelectric converter. This unit divides a two-dimensional image into picture elements in a vertical or horizontal direction and treats a line or column of one-dimensional images as an input signal as well as an output signal. The unit performs sequential calculations on each line (or column) of and lines (or columns) of pixels of the two-dimensional image thereby causing the problem of time-consuming operations.
The device according to the present invention comprises a two-dimensional spatial light modulator (e.g. a liquid crystal panel) and a two-dimensional photoelectric converter (e.g. a charge-coupled device) instead of the one-dimensional spatial light modulator and the one-dimensional photoelectric converter of the prior art. This device treats a two-dimensional image itself as an input signal as well as an output signal and can execute a discrete cosine transformation of the two-dimensional image itself at one time thereby ensuring high-speed processing.
The publication of unexamined patent application, JP, A, 2-127625 discloses a unit for light discrete cosine transformation and an image-coding device using a unit which is basically composed of a two-dimensional photoelectric converter, a first-optical system for sampling real parts of a two-dimensional Fourier transformation, a-second optical system for sampling cosine terms and a photoelectric converter. The device is intended to execute a cosine transformation by using a Fourier transform lens for effecting light wave transformation.
In contrast to the above mentioned prior art, the device according to the present invention, does not require complex optical systems for sampling real parts of a two-dimensional Fourier transformation and for sampling cosine terms and can execute a discrete cosine transformation on the basis of ON- and summations by means of a converging lens and a mask (spatial filter) which substitutes a basic pattern of a discrete cosine transformation by a variety of transmission.
As mentioned above, the conventional pictorial data encoding/decoding system utilizes the coding method based on "discrete cosine transform", which is featured by the relatively low compressibility of the information and complicated structure of the processing system (since a picture is divided into blocks), which neighbours are generally related with each other and which are separately processed. Since the block processing also causes a restored picture to have block separating lines thereon, it is necessary to remove said lines from the picture by using a post-processing filter which is very hard to design and manufacture, and which has a high quality and excellent performance, because of its dependence upon the picture content.